Effects of Land Use Types on CH₄ and CO₂ Production Potentials in Subtropical Wetland Soils

Changes in land use types can alter the soil and environmental characteristics of wetlands, which in turn influence the magnitude of greenhouse gas production by soil microbes. However, the effects of land use change on the production potential of methane (CH₄) and carbon dioxide (CO₂) in subtropical wetland soils and the underlying controls are still largely unknown. In this study, we examined the soil CH₄ and CO₂ production potentials under five different land use types (natural mangrove, Gei Wai water channel, Gei Wai forest, reedbed, and freshwater pond) and their relationships with soil physico-chemical properties in a subtropical wetland in Hong Kong using aerobic and anaerobic laboratory incubation experiments. Our results showed an overall decreasing trend of CH₄ and CO₂ production potentials down the soil profile at all sites, which could be attributed to a reduction in the concentrations of soil organic matter (SOM), total Kjeldahl nitrogen (TKN) and ammonium nitrogen (NH₄⁺-N). Moreover, the soil CH₄ and CO₂ production potentials varied significantly in the surface soils among land use types, but were more similar across the sites in the deeper soils. The conversion of natural mangrove to other land use types significantly reduced both the aerobic and anaerobic CO₂ production potentials in the top 10 cm soils, except for Gei Wai forest, which demonstrated significantly higher CO₂ production rates (61.15–97.91 μg g⁻¹ day⁻¹). Meanwhile, the mean CH₄ production potential in the surface soils of natural mangrove (0.05 μg g⁻¹ d⁻¹) was significantly lower than that in the Gei Wai forest and Gei Wai channel (0.26–0.27 μg g⁻¹ day⁻¹) but slightly higher than that in the freshwater pond and reedbed (0.00–0.02 μg g⁻¹ day⁻¹). The high soil CH₄ and CO₂ production potentials observed in the Gei Wai forest could be explained by the high soil concentrations of SOM, TKN and NH₄⁺-N. On the other hand, the lower anaerobic CH₄ and aerobic CO₂ productions observed in the reedbed could be attributed to the lower concentrations of NH₄⁺-N and available phosphorus. Our findings highlighted the significant impacts of land use types on the CH₄ and CO₂ production potentials of subtropical wetland soils, which had practical implications for wetland management for climate change mitigation.